The principal investigator has developed a quantitative radiometric assay for skeletal muscle myosin (16), capable of using monoclonal antibodies that can distinguish between closely related myosin isozymes. The assay will be used to develop quantitative means of identifying abnormal muscle performance and abnormalities of myosin isozyme composition, e.g., elevated fetal myosin (9) or abnormal Type I/Type II myosin ratios (8). Specifically, we will investigate the following areas: 1) We will pursue our original goal of refining the well-known relationship of myosin ATPase activity (a function of myosin isozyme composition) to contractile speed (1,6). We hypothesize that the normal range of in vitro contractile properties for mixed fiber type, small muscle bundles can be narrowed significantly by correlating observed contractile properties with exact myosin isozyme compositions. This will allow abnormalities of contractile properties to be more easily identified. 2) We will test the hypothesis that the maximum specific isometric tension (P0) of skeletal muscle is directly proportional to the amount and type of myosin present in the muscle. The maximum specific tetanic tension is usually calculated by assuming that the functional cross-sectional area is some function of the mass divided by resting length. This technique of calculating P0 has many disadvantages in that it assumes that a fixed fraction of the muscle mass is involved in force generation, an assumption that is probably not true in regenerating or transplanted muscle, and may not be true for Type I vs Type II muscle fibers. Studies involving measurement of P0 will be facilitated by the development of a technique which is valid over a wider range of muscle states. 3) We will characterize an approximately 100K Dalton molecular weight myosin immunoreactive fragment that we have identified with our assay in extracts of normal and denervated muscle. This protein does not appear to be an artifact of the preparation procedure and appears to be increased in muscle that has been recently denervated. Verification of such a fragment as a degradation fragment of myosin could lead to significant advances in understanding the process of in vivo myosin degradation.